Polymers have been demonstrated to have utility in methods of forming complex or intricately shaped parts from ceramic powders. The forming of ceramics is important because machining ceramics into complex shapes is time consuming and expensive, and in many cases impractical. Strivens, U.S. Pat. No. 2,939,199, discloses a method of forming articles from ceramic powders wherein the ceramic powders are mixed with a vehicle comprising a thermosetting material and a plasticizer, and the resultant mixture is injection molded into a mold of a desired shape and heated to cure the thermosetting component. The vehicle is then removed from the molded part by low pressure distillation or by solvent extraction. Kingery et al., U.S. Pat. No. 3,351,688, discloses a method wherein the ceramic powder is mixed with a binder such as paraffin at a temperature where the binder is liquid, and the resulting mixture is cast into a mold of the desired shape. The binder is permitted to solidify so that a green piece is formed having a uniform density. Curry, U.S. Pat. No. 4,011,291, and Ohnsorg, U.S. Pat. No. 4,144,297, disclose the use of a paraffin wax binder for molding ceramic powders into desired shapes. Rivers, U.S. Pat. No. 4,113,480, and Wiech, Jr., U.S. Pat. No. 4,197,118, disclose methods for molding parts from metal powders by mixing the powders with binder materials and injection molding the resultant mixtures. Additional methods which employ binder materials are disclosed by Hurther et al., U.S. Pat. No. 4,478,790, and Kato, U.S. Pat. No. 4,460,527.
It is known that gelcasting can also be a useful way of forming ceramic materials. Gelcasting is a method of molding ceramic powders into green products wherein a monomer solution is used as a binder vehicle and the controlled polymerization of the monomer in solution serves as a setting mechanism. The resulting green product is of exceptionally high strength and may be dried to remove water. After drying, the product may be further heated to remove the polymer and may also subsequently be fired to sinter the product to a high density. Gelcasting methods are disclosed in Janney, U.S. Pat. No. 4,894,194, Janney et al, U.S. Pat. No. 5,028,362, and Janney et al., U.S. Pat. No. 5,145,908. Gelcasting of ceramics such as alumina is described by A. C. Young, O. O. Omatete, M. A. Janney, and P. A. Menchhofer, "Gelcasting of Alumina," J. Am. Ceram. Soc., 74 [3] 612-18 (1991). Mark A. Janney, Weiju Ren, Glen H. Kirby, Stephen D. Nunn, and Srinath Viswanathan, "Gelcast Tooling: Net Shape Casting and Green Machining," Materials and Manufacturing Processes, 1997 describe the use of a water-based gelcasting system to form parts using H13 tool steel powder. R. Raman, M. A. Janney, and S. Sastri, "An Innovative Processing Approach to Fabricating Fully Dense, Near-Net-Shape Advanced Material Parts," Advances in Powder Metallurgy and Particulate Materials, 1996, Metals Powder Industries Federation, Princeton, N.J., 1996 describe the use of a water-based gelcasting system to form parts using an 83/17 aluminum/silicon alloy powder. S. D. Nunn, J. O. Kiggans, Jr., R. E. Simpson, II, and J-P Maria, "Gelcasting of Silicon Compositions for SRBSN," Ceram. trans., 62, 255-62 (1996) describe the use of an alcohol-based gelcasting system and a water-based gelcasting system to form parts using silicon powder. M. A. Janney, "Gelcasting Superalloy Powders," in P/M in Aerospace, Defense and Demanding Applications--1995, Metals Powder Industries Federation, Princeton, N.J., 1995, describes the use of a water-based gelcasting system to form parts. The disclosures of these references are incorporated fully by reference.
Gelcast ceramic bodies have been demonstrated to be machinable in the "green" state, after drying and before firing. See S. D. Nunn, O. O. Omatete, C. A. Walls, and D. L. Barker, "Tensile Strength of Dried Gelcast Green Bodies," Ceram. Eng. Sci. Proc., 15 [4] 493-498 (1994), and S. D. Nunn and G. H. Kirby, "Green Machining of Gelcast Ceramic Materials," Ceram. Eng. Sci. Proc., 17 [3-4] (1996).
Hydrogels comprise a three-dimensional polymer network and water. Polymers in hydrogels are characterized by hydrophilicity and insolubility in water. The presence of water-solubilizing groups, such as --OH, --COOH, --CONH.sub.2, --CONH--, --SO.sub.3 H and the like, render the polymer hydrophilic. The three-dimensional polymer network creates the stability and insolubility of the hydrogel. In the presence of water, the hydrogel will swell to an equilibrium volume, which results from the balance between the disbursing forces acting on hydrated chains and cohesive forces that do not prevent the penetration of water into the network. The cohesive forces may be due to covalent cross-linking, as well as electrostatic, hydrophobic, or dipole-dipole interactions. The tacticity and crystallinity of the polymer, and the degree and nature of cross-linking, are responsible for the characteristics of the hydrogel in the swollen state.
Hydrogels for use in gelcasting are typically formed by co-polymerizing a monomer with a cross-linking agent. The monomer is typically monofunctional and the cross-linking agent is multifunctional, the functional group typically being vinyl or allyl. Typical monomers include acrylamide, methacrylamide, N-vinyl pyrrolidone, hydroxyalkyl methacrylates, hydroxyalkyl acrylates, acrylic acid and methacrylic acid. Combinations of monomers are also utilized. Cross-linking agents include methylene bisacrylamide, and diacrylates and dimethacrylates. Combinations of cross-linking agents are also utilized. The chemistry for forming hydrogels requires initiators to polymerize the system. The acrylamide and methacrylamide monomers are solids, and time is required to dissolve these solids. The cross-linking agents are also slow to dissolve, and are higher-cost components compared to the monomers.